SUMMARY/ABSTRACT Combination antiretroviral therapy (ART) can control but not cure HIV infection because of reservoirs, particularly latently infected CD4 T cells, established before ART was begun, from which infection rebounds if ART is interrupted. To achieve a functional cure or eradication of this reservoir, ?shock and kill? strategies seek to reactivate latently infected CD4+ T-cells for elimination by immune or other mechanisms, but current methods for reactivation predicated on the concept that harbor HIV proviruses that have been transcriptionally silenced by epigenetic or other mechanisms have proven quite inefficient in reversing latency. This proposal describes a new approach to reactivating latently infected cells by lentivirus vector induced expression of the HIV tat gene. Preliminary results provide evidence of extraordinary efficiency of Tat-reactivation and the underpinnings for an in vitro latency reactivation assay with new single cell measurements of the latently infected cell reservoir from which infection will rebound if ART is interrupted. The Specific Aims of the proposal are to 1) further develop the Tat-reactivation assay as a faster and more accurate assay for reservoir evaluations in peripheral blood (PB) and lymphoid tissues (LT); determine the correlations between single cell measurements of virus producing cells, cells with intact HIV genomes and current quantitative virus growth assays; and 2) apply the assay to assess the impact of ART in very early stage infection to limit the size of HIV reservoirs in PB and LT. peripheral blood (PB) and lymphoid tissues (LT). The long-term goal of investigation of Tat-reactivation is development of more effective approaches to the ?shock? component of shock and kill approaches to reducing HIV reservoirs.